There has already been described, i.a. in the patent literature, a large number of processes for preparing various petroleum fractions from various fossil fuels.
According to one of these processes hydrocarbon mixtures, including high octane gasoline, are prepared directly from lower alcohols and/or corresponding ethers by the catalytic reaction over synthetic zeolite catalysts. The process and the catalysts are described, i.a., in the U.S. Pat. Nos. 3,702,886, 3,709,979, 3,832,449, 3,899,544, and 3,911,041. The alcohols and/or ethers usable as starting materials may have been prepared in separate plants. It has been found advantageous, however, to integrate the process for the preparation of hydrocarbons by means of zeolite catalysts with a process for preparing the said alcohols and/or ethers from a suitable raw material such as natural gas or coal. Such integrated processes have already been described and especially two of them have been the subject of interest. In one of them a synthesis gas containing hydrogen and carbon oxides is converted via methanol (MeOH) into hydrocarbons, and in the other the conversion into hydrocarbons takes place via methanol/dimethyl ether (MeOH/DME).
Such a process for conversion via MeOH is described, e.g. in German Patent Publication DE-OS No. 2,846,693. In the process according to the German Patent Publication a gas mixture mainly consisting of carbon oxides and hydrogen is reacted in a first step using a methanol synthesis catalyst to produce an intermediate product containing methanol. The entire intermediate product is then further converted in a second step using a zeolite catalyst to produce a product stream containing hydrocarbons. The product stream is cooled and a fraction of the stream consisting of hydrocarbons containing at least 5 C-atoms per molecule is separated while the remaining part of the product stream is recycled to the inlet of the first step. The second step of the process preferably is carried out in a cooled reactor at a temperature of 250.degree.-400.degree. C.
A process for conversion via MeOH/DME is described e.g. in U.S. Pat. No. 3,894,102. In the process according to that specification a mixture of carbon monoxide and hydrogen is contacted with a catalyst mixture consisting of a methanol synthesis catalyst and an acidic dehydration catalyst in a first step to produce an intermediate product having a high content of DME. The intermediate product or a part thereof is thereafter reacted in a second step over a zeolite catalyst to produce a product containing high octane gasoline.
Irrespective of which of the described known process routes is employed it has in practice not been possible to carry out the zeolite catalyzed process step without process-related problems as described below.
The preparation of hydrocarbons by the conversion of MeOH and/or DME takes place by exothermal reactions and is conducted using controlled temperature conditions. Thus, according to the known technique the temperature is controlled by using cooled reactors or by using one or more adiabatic reactors optionally combined with recycling of a fraction of the product gas to limit the adiabatic temperature increase.
It is important to control the temperature considering the catalyst as well as the composition of the product obtained.
Thus, using a relatively low temperature results in a slow deactivation of the catalyst but at the same time a product of poor quality is obtained (high content of undesirable components, especially durene).
On the other hand a relatively high temperature results in a fast deactivation of the catalyst but at the same time a product of good quality is obtained.
The deactivation is caused by the formation of high molecular weight compounds and possibly carbon which is deposited on the surface and in the pore system of the catalyst, thereby blocking the active centers of the catalyst resulting in reduced catalytic activity.
Hitherto it has not been possible to obtain at the same time a slow deactivation and the desired product composition. Therefore it has been necessary to carry out
(1) frequent regeneration/replacement of the catalyst or PA0 (2) further treatment of the product to obtain an end product of the desired composition.